1. Field of the Invention
The field of art to which this invention pertains is the separation of a component from a fluid mixture of components.
2. Description of the Prior Art
There are many separation processes known for separating components from a fluid mixture. Fractional distillation or crystallization are examples of means ideal for separating liquid mixtures of components having different boiling points or freezing points, respectively. Gas or liquid chromatography makes use of material or adsorbents having varying degrees of affinity for different components of a fluid mixture, thereby causing the components to separate as they flow through the material. Similarly, materials known as molecular sieves may affect the rates at which each component of a fluid mixture passes through them by admitting only molecules of certain of the components into the pore structure of the material, but not other components, thus the component for which the material in question has the greater affinity or retention capacity may be recovered or "desorbed" by means of a desorbent material.
A very successful process for separating components from a feed mixture based on the use of adsorbents or molecular sieves for chromatographic type separations is the countercurrent moving-bed or simulated moving-bed countercurrent flow system. In the moving-bed or simulated moving-bed processes the adsorption and desorption operations are continuously taking place which allows both continuous production of an extract (more selectively adsorbed component) and a raffinate (less selectively adsorbed component) stream and the continual use of feed and desorbent streams. The operating principles and sequence of such a flow system are described in U.S. Pat. No. 2,985,589 to Broughton et al. In that system it is the progressive movement of multiple liquid access points down an adsorbent chamber that simulates the upward movement of adsorbent contained in the chamber. Only four of the access lines are active at any one time: the feed input stream, desorbent inlet stream, raffinate outlet stream, and extract outlet stream access lines. Coincident with this simulated upward movement of the solid adsorbent is the movement of the liquid occupying the void volume of the packed bed of adsorbent. So that countercurrent contact is maintained, a liquid flow down the adsorbent chamber may be provided by a pump. As an active liquid access point moves through a cycle, that is, from the top of the chamber to the bottom, the chamber circulation pump moves through different zones which require different flow rates. A programmed flow controller may be provided to set and regulate these flow rates.
The active liquid access points effectively divide the adsorbent chamber into separate zones, each of which has a different function. It is generally necessary that three separate operational zones be present in order for the process to take place although in some instances an optional fourth zone may be used.
The adsorption zone, zone 1, is defined as the adsorbent located between the feed inlet stream and the raffinate outlet stream. In this zone, the feed stock contacts the adsorbent, an extract component is adsorbed, and a raffinate stream is withdrawn. Since the general flow through zone 1 is from the feed stream which passes into the zone to the raffinate stream which passes out of the zone, the flow in this zone is considered to be a downstream direction when proceeding from the feed inlet to the raffinate outlet streams.
Immediately upstream with respect to fluid flow in zone 1 is the purification zone, zone 2. The purification zone is defined as the adsorbent between the extract outlet stream and the feed inlet stream. The basic operations taking place in zone 2 are the displacement from the non-selective void volume of the adsorbent of any raffinate material carried into zone 2 by the shifting of adsorbent into this zone and the desorption of any raffinate material adsorbed within the selective pore volume of the adsorbent or adsorbed on the surfaces of the adsorbent particles. Purification is achieved by passing a portion of extract stream material leaving zone 3 into zone 2 at zone 2's upstream boundary, the extract outlet stream, to effect the displacement of raffinate material. The flow of material in zone 2 is in a downstream direction from the extract outlet stream to the feed inlet stream.
Immediately upstream of zone 2 with respect to the fluid flowing in zone 2 is the desorption zone or zone 3. The desorption zone is defined as the adsorbent between the desorbent inlet and the extract outlet stream. The function of the desorption zone is to allow a desorbent material which passes into this zone to displace the extract component which was adsorbed upon the adsorbent during a previous contact with feed in zone 1 in a prior cycle of operation. The flow of fluid in zone 3 is essentially in the same direction as that of zones 1 and 2.
In some instances an optional buffer zone, zone 4, is utilized. This zone, defined as the adsorbent between the raffinate outlet stream and the desorbent inlet stream, if used, is located immediately upstream with respect to the fluid flow to zone 3. Zone 4 would be utilized to conserve the amount of desorbent utilized in the desorption step since a portion of the raffinate stream which is removed from zone 1 can be passed into zone 4 to displace desorbent material present in that zone out of that zone into the desorption zone. Zone 4 will contain enough adsorbent so that raffinate material present in the raffinate stream passing out of zone 1 and into zone 4 can be prevented from passing into zone 3 thereby contaminating extract stream removed from zone 3. In the instances in which the fourth operational zone is not utilized the raffinate stream passed from zone 1 to zone 4 must be carefully monitored in order that the flow directly from zone 1 to zone 3 can be stopped when there is an appreciable quantity of raffinate material present in the raffinate stream passing from zone 1 into zone 3 so that the extract outlet stream is not contaminated.
A cyclic advancement of the input and output streams through the fixed bed of adsorbent can be accomplished by utilizing a manifold system in which the valves in the manifold are operated in a sequential manner to effect the shifting of the input and output streams thereby allowing a flow of fluid with respect to solid adsorbent in a countercurrent manner. Another mode of operation which can effect the countercurrent flow of solid adsorbent with respect to fluid involves the use of a rotating disc valve in which the input and output streams are connected to the valve and the lines through which feed input, extract output, desorbent input and raffinate output streams pass are advanced in the same direction through the adsorbent bed. Both the manifold arrangement and disc valve are known in the art. Specifically rotary disc valves which can be ultilized in this prior art operation can be found in U.S. Pat. Nos. 3,040,777 to Carson et al and 3,422,848 to Liebman et al. Both of the aforementioned patents disclose a rotary type connection valve in which the suitable advancement of the various input and output streams from fixed sources can be achieved without difficulty.
In many instances, one operational zone will contain a much larger quantity of adsorbent than some other operational zone. For instance, in some operations the buffer zone can contain a minor amount of adsorbent as compared to the adsorbent required for the adsorption and purification zones. It can also be seen that in instances in which desorbent is used which can easily desorb extract material from the adsorbent that a relatively small amount of adsorbent will be needed in a desorption zone as compared to the adsorbent needed in the buffer zone or adsorption zone or purification zone or all of them. Since it is not required that the adsorbent be located in a single column, the use of multiple chambers or a series of columns is within the scope of the process.
It is not necessary that all of the input or output streams be simultaneously used, and in fact, in many instances some of the streams can be shut off while others effect an input or output of material. The apparatus which can be utilized to effect this prior art process can also contain a series of individual beds connected by connecting conduits upon which are placed input or output taps to which the various input or output streams can be attached and alternately and periodically shifted to effect continuous operation. In some instances, the connecting conduits can be connected to transfer taps which during the normal operations do not function as a conduit through which material passes into or out of the process.
It is usually essential to the prior art simulated moving-bed process that at least a portion of the extract output stream will pass into a separation means wherein at least a portion of the desorbent material can be separated to produce an extract product containing a reduced concentration of desorbent material. Preferably at least a portion of the raffinate output stream will also be passed to a separation means wherein at least a portion of the desorbent material can be separated to produce a desorbent stream which can be reused in the process and a raffinate product containing a reduced concentration of desorbent material. The separation means will typically be a fractionation column, the design and operation of which is well known to the separation art.
Further reference can be made to D. B. Broughton U.S. Pat. No. 2,985,589, and to a paper entitled "Continuous Adsorptive Processing--A New Separation Technique" by D. B. Broughton presented at the 34th Annual meeting of the Society of Chemical Engineers at Tokyo, Japan on April 2, 1969, for further explanation of the simulated moving-bed counter-current process flow scheme.
There have been other flow schemes since the above basic Broughton et al invention which are also based in some manner or chromatographic separation of feed stream components through the establishment of a concentration gradient of such components in a bed or beds of adsorbent material exhibiting adsorptive selectivity for one component over another. For example, Japanese Public Disclosure 118400/80 (Public Disclosure Date Sept. 11, 1980) of Miyahara et al discloses the use of a single (non-simulated moving-bed) column of ion exchange resin with an inlet at the top and an outlet at the bottom for the separation of glucose from fructose by sequentially passing into the column, in the appropriate order, the feed stream, the desorbent stream and various effluent streams held in intermediate storage, each stream being introduced at the appropriate time with relation to the concentration gradient in the column. Similarily, in the process of U.S. Pat. No. 4,267,054 to Yoritomi et al a concentration gradient is established in one or more columns (simulated moving-bed) with the discontinuous and intermittent introduction of the feed and desorbent streams which cause disturbance of the gradient and the introduction of various recycle streams direct from the column effluent (rather than intermediate storage) as appropriate to re-establish the concentration gradient. Other examples of processes involving flow schemes similar to any of the above art, but no more relevant to the present invention, are as set forth in U.S. Pat. Nos. 3,205,166 to Ludlow et al (actual moving bed); 3,291,726 to Broughton; 3,310,486 to Broughton et al; 3,416,961 to Mountfort et al; 3,455,815 to Fickel; 3,686,117 to Lauer et al; 3,715,409 to Broughton; 4,155,846 to Novak et al; 4,157,267 to Odawara et al; 4,022,637 to Sutthoff et al; 4,031,155 to Healy et al; and 4,332,623 to Ando et al.
In contradistinction to the above discussed prior art, the present invention achieves chromatographic separation of components of a feed mixture by the employment of a simulated moving-bed co-current flow system without, inter alia, the purification or buffering zones such as in Broughton et al (U.S. Pat. No. 2,985,589), the intermediate storage such as in Miyahara et al or the discontinuous and intermittent characteristics such as in Yoritomi et al.